In radio frequency ion trap technology, a three-dimensional ion trapping using a radio frequency quadrupole field (so called Paul trap), and a linear ion trapping using a two-dimensional radio frequency quadrupole field and (a direct current voltage are known. This Paul trap comprises a ring electrode, and two end cap electrodes facing toward the hole in the ring. A radio frequency voltage is applied between the ring electrode and two end cap electrodes so as to generate a 3-dimensional radio frequency quadrupole electric field between the electrodes in which ions accumulate.
A description of this method of accumulating ions is given for example in H. G. Dehmelt, Adv.At.Mol.Phys. 3, 53 (1967).
As shown for example in U.S. Pat. No. 4,755,670 (1988), M. G. Raizen et al: Phys. Rev. A45, 6493 (1992), and J. D. Prestage et al: J. Appl. Phys. 66 1013 (1989), a linear quadrupole radio frequency electric field is generated in the vicinity of the center of the electrodes by applying a radio frequency electric field to the linear quadrupole electrode structure such that the electrodes on opposite sides have the same phase, and ions are thereby stably trapped in the direction perpendicular to the long axis of the electrodes. However, in this situation, ions leak from the ends of the electrodes. This is prevented by applying a direct current voltage having the same polarity of the trapped ions to the ends of the electrodes.
One field of application of ion trapping technology in industry is that of mass spectrometry. A mass spectrometer using a Paul trap, i.e. an ion trap mass spectrometer, is introduced in U.S. Pat. No. 2,939,952 invented by Paul et al in 1960. However, at that time an effective operation method for mass spectrometry was not given, and due to its low resolution and narrow mass range for mass analysis, it did not lead to its practical use as a mass spectrometer. When the operating method disclosed in U.S. Pat. No. 4,540,884, "mass selective instability", was invented, the device reached a practical level of mass range, detection sensitivity and detection resolution. However, mass spectrometry devices using linear ion trapping are not currently in practical use. A method of using these devices for mass spectrometry was suggested in U.S. Pat. No. 4,755,670 (1988). According to this method, the ions which accumulate in the trap are made to resonate in a mass-dependent oscillation mode, and the oscillation is detected electrically. Considering the induced signal strength, it may be expected that the sensitivity will be low. An example of a mass-analyzing function of a curved linear ion trap with an external ion detector is given by Waki et al in Physical Review Letters, Vol. 68, page 2007-2010 (1992), where the ion detector detects trapped ions that have been elected perpendicularly to the center axis of the trap after undesired ions are ejected using mass-selective instability. A similar configuration with other types of linear traps in combination with aforesaid techniques used in Paul traps is described by Bier et al in U.S. Pat. No. 5,420,425 (1995).
When one attempts to improve the sensitivity of the mass spectrometry device using the Paul trap which is now being put to practical use, an adverse effect appears due to background ions. In other words, the detection sensitivity of ions to be detected deteriorates when there is a large amount of background ions. This effect must therefore be removed. One method of doing this is the method of operating an ion trap mass spectrometer introduced, for example in U.S. Pat. No. 5,134,286. Therein it is proposed that background ions are mass-selectively ejected during injection of ions into the ion trap and in the stage prior to performing mass analyzing. However, according to this method, there are following disadvantages in removing the background ions in the ion trap while they are brought to resonance by supplying them with energy, which interferes with a high sensitive analysis. Firstly, during background ion removal, background ions which are brought to resonance collide with out of the trap electrodes. Secondly, background ions having a large kinetic energy collide with sample ions that are trapped, and the sample ions are thereby destroyed. Thirdly, the ion detector and the trap electrodes are contaminated by the large amount of background substances, and detection sensitivity and mass resolution fall.
To deal with these problems, the background ions may be removed using a mass filter before they enter the ion trap. One example of this is disclosed in, for example, K. L. Morand et al: International Journal of Mass Spectrometry and Ion Processes 105 13 (1991). This prior art example describes a mass spectrometer wherein a mass filter is connected in cascade with a mass analyzer comprising essentially a Paul trap. After the mass filter has removed background ions to increase the purity of the sample ions, the sample ions enter a hole in an end cap electrode of the Paul trap, and accumulate in the trap. The ions are then analyzed in the mass analyzer. According to this prior art, the ions trapped in the mass analyzer contain almost no background. Therefore, loss or destruction of ions to be detected due to collisions with background ions is suppressed. Further, there is no contamination of the ion trap electrodes and the ion detector by background ions.
However, this mass spectrometer comprising a mass filter and a mass analyzer comprising essentially a Paul trap has a disadvantage that, as the ion trapping efficiency is low, it is difficult to obtain higher sensitivity. This is due to the fact that the mass filter has a linear construction whereas the Paul trap has a 3-dimensional construction. Specifically, a high kinetic energy must be given to the incident ions so that they can pass through the mass filter and into the Paul trap. The sample ions therefore can collide with the end cap electrode opposite to the entrance hole, and can be lost. To prevent this, the DC potential of the opposite electrode is increased, both potentials being restored after the ion injection so that the ions are trapped inside the trap. This causes an intermittent ion injection. Hence, the number of sample ions which can be trapped on each mass analysis operations is low and the sensitivity cannot be improved. Another possible method is to slow down the ions by collision with a gas so that they are stopped inside the ion trap. In general, an ion trap mass spectrometer is set in a helium gas environment ranging from 10.sup.-1 to 10.sup.-6 Torr so as to improve the sensitivity. It might be thought that this helium gas could be used to stop the ions with high frequency. However, it is difficult to efficiently stop sample ions, that have passed through the mass filter with high kinetic energy, using dilute gas.